1. Field of the Invention
The present invention generally relates to a switching regulator control circuit. More specifically, the present invention is directed to a regulator control circuit capable of avoiding such a fact that when an impedance of an input power supply of a switching regulator is increased, a switch element is continuously turned ON and thus, a large current flows through a power supply and the switch element to thereby break down this switch element.
2. Description of the Related Art
FIG. 6 is a circuit diagram for representing one of the conventional switching regulator (SW regulator) control circuits. That is, there is provided an error amplifying circuit 13 which amplifies a difference voltage between a reference voltage "Vref" of a reference voltage circuit 10 and a voltage "Va" appeared at a junction point between a bleeder resistor 11 and a bleeder resistor 12. The bleeder resistors 11/12 sub-divide an output voltage "Vout" of an output terminal 2 of an SW regulator. Assuming now that the output voltage of the error amplifier circuit 13 is "Verr", the output voltage of the reference voltage circuit 10 is "Vref", and the voltage appeared at the junction point between the bleeder resistor 11 and the bleeder resistor 12 is "Va", if Vref&gt;Va, then the output voltage "Verr" of the error amplifier circuit 13 is increased, whereas if Vref&lt;Va, then this output voltage "Verr" is decreased. A pulse width control circuit 14 which enters the output voltage "Verr" of the error amplifying circuit 13 as an input signal controls the ON time of the switch element (SW element) and the OFF time thereof in response to a value of this output voltage "Verr". The SW element is connected to an SW element drive circuit 16 so as to be turned ON/OFF.
The SW element drive circuit 16 is operated while the output voltage "Vout" is used as the power supply. The reference voltage circuit 10, the error amplifying circuit 13, and the pulse width control circuit 14 are operated while the voltage "Vin" of the input terminal 1 is used as the power supply. When a power MOS transistor is employed, for instance, as the SW element of the SW element drive circuit, if a high drive voltage (namely, gate-to-source voltage) is used, then the ON resistance of this power MOS transistor may be lowered. As a result, when the SW element drive circuit is driven by employing the boosted output voltage "Vout", the efficiency of the SW regulator may be increased. A level shifter (will be referred to as an "L/S" hereinafter) 15 is employed between the pulse width control circuit 14 and the SW element drive circuit 16, and converts a signal derived from the pulse width control circuit 14 of the Vin-power supply system into a signal level of the SW element drive circuit 16 of the Vout-power supply system having the different power supply voltage from that of the Vin-power supply system.
FIG. 7 shows an example of a step-up type SW regulator. In this SW regulator, both a coil 21 and an SW regulator control circuit 30 are connected to an input power supply 20. A rectifying element 23 is connected between the coil 21 and an output capacitor 24. A load 25 is connected parallel to the output capacitor 24. In general, an output impedance 26 of the input power 20 is low, and therefore is negligible. However, in the case that an extraordinary condition happens to occur in the input power supply 20, this output impedance 26 will have a certain impedance value. Also, when a cell and the like are employed as the input power supply 20, the input power supply 20 will have an impedance value of approximately several .OMEGA. to ten .OMEGA..
FIG. 8 indicates a waveform produced when the power supply is turned ON in such a case that the impedance 26 of the input power supply 20 of FIG. 7 is negligibly small. FIG. 8(a) shows both a voltage "V20" of the power supply 20 of FIG. 7 and a power supply voltage "Vin" of the SW regulator control circuit 30, and FIG. 8(b) represents an output voltage "Vout" of the SW regulator. In these drawings, abscissas denote time. Since the impedance 26 of the input power supply 20 is negligibly small, the waveform of "V20" is overlapped with the waveform of "Vin" in FIG. 8(a). The reason why the output voltage "Vout" of FIG. 8(b) is gradually increased is caused by a soft starting function of the SW regulator control circuit. This soft starting function is such a function that the output voltage is gradually increased in order that an overshoot phenomenon is not produced in the output voltage "Vout" when the power supply is turned ON. This soft starting function is not described in this specification.
FIG. 9 shows a waveform produced when the power supply is turned ON in the case that the impedance 26 of the input power supply 20 of FIG. 7 is on the order of several .OMEGA.. FIG. 9(a) shows a voltage "V20" of the input power supply 20 and a power supply voltage "Vin" of the SW regulator control circuit 30 in FIG. 7, FIG. 9(b) represents an output voltage "Vout" of the SW regulator, and FIG. 9(c) denotes a current "I20" of the input power supply 20. In FIG. 9(a) to FIG. 9(c), abscissas show time. When a current flows through the input power supply 20 by the impedance 26 of the input power supply 20, the power supply voltage "Vin" of the SW regulator control circuit is decreased. In FIG. 9, while the SW regulator is operated in the step-up operation, a current flows through the input power supply 20. As a result, the input voltage "Vin" of the SW regulator control circuit 30 is decreased lower than, or equal to the operation voltage of the SW regulator control circuit 30, so that the SW regulator control circuit 30 cannot be operated under normal condition. Thus, FIG. 9 represents such a condition that the output of the SW element drive circuit continuously turns ON the SW element. For example, in such a case that the value of the output voltage "V20" of the input power supply 20 is 2 V, the value of the output impedance 26 is 1.5 .OMEGA., and a current of 1 A flows through the input power supply 20 when the power supply is turned ON, the input voltage "Vin" of the SW regulator control circuit 30 is decreased up to 0.5 V. Assuming now that the minimum operation voltage of the SW regulator control circuit 30 is selected to be 1 V, the SW regulator control circuit 30 cannot be operated in the normal mode under this low-voltage condition, and also the output of the L/S 15 of FIG. 6 becomes uncertain. As a result, when the voltage of the EXT terminal of the output of the SW element drive circuit 16 is stopped under such a condition that the SW element 22 of FIG. 7 is turned ON, a large current continuously flows through the input power supply 20, the coil 21, and the SW element 22. Thus, there is such a risk that these circuit elements are deteriorated, and will be broken down in the worst case.
However, in the conventional SW regulator, when the output impedance of the input power supply is increased, the following problem will occur. That is, while the SW regulator is operated under step-up operation, the power supply voltage of the SW regulator control circuit is lowered, the SW regulator control circuit cannot be operated under normal condition, and the SW element is continuously turned ON, so that the large current flows through the power supply circuit and the SW element, which may give damages to these circuit elements.